It has been recognized for some time that most of the integrated circuit devices, processes and techniques that revolutionized the electronics industry can be adapted to produce optoelectronic integrated circuits. Due to the dominance of silicon as the material of choice for electronic circuits, the use of silicon-based optoelectronic integrated circuit platforms is highly desirable. In many of today's optoelectronic circuit arrangements, a Silicon-On-Insulator (SOI) wafer is utilized as the platform, with one or more silicon waveguides being formed in the upper silicon layer of the SOI structure. These high-index contrast silicon waveguides (with respect to the refractive index value of the insulator material) permit strong light confinement and the use of relatively tight bends in the waveguide topology, as well as the miniaturization of the various electro-optical components used with the silicon waveguides. As the complexity level increases, integration of more functions and components within a single package is required to meet system-level requirements and reduce the associated size and cost of the complete system. A clear advantage of using silicon-based optoelectronic integrated circuits stems from the fact that many required tools, techniques and processes have already been developed in silicon to meet the needs of conventional electronics. In addition, the material costs of silicon-based devices are considerably lower than those for competing technologies, such as gallium arsenide or indium phosphide.
At the present time, it is not possible to monolithically integrate light sources with the remaining components on the opto-electronic platform, since silicon-based lasers and light emitting diodes (LEDs) are only now beginning to be developed. Thus, the light signal must be introduced to the silicon waveguide from an external source.
One conventional prior art arrangement for introducing light into a silicon waveguide is to use a separate laser or LED module emitting a free space beam, followed by optical elements to shape, focus and steer the light beam and/or adjust its polarization state. Alternatively, a fiber-connected light source module can be used, where the coupling termination of the fiber is then followed by similar light coupling components to focus the light signal into the silicon waveguide. While receiving elements may be incorporated in the silicon wafer as on-chip or integrated detectors, there are many applications where the user will need direct access to the optical signal after the on-chip functions have been performed. Thus, it is appropriate to provide an optical output port that would generally be a fiber-based termination, although the preferred embodiments do not exclude other output configurations.
A common prior art technique for coupling light from an external source to a silicon waveguide is to cleave end facets on both the waveguide and the mating fiber termination. Examples of fiber terminations include, but are not limited to, multimode or single-mode fibers with small or zero cleave angles, and specially-shaped or lensed single-mode fibers that produce spot sizes as small as 1.5 μm. The fiber termination is aligned to allow maximum light transmission through the waveguide, and then fixed in position. Anti-reflection (AR) coatings can be used on both the fiber termination and the waveguide facet to reduce the Fresnel losses. Since input and output ports for devices must be located at edge facets of the waveguide-containing wafer die for this configuration, significant restrictions on device geometry (e.g., topology and/or size) are imposed by using this prior art edge coupling constraint.
The above-described edge coupling technique is effective if the mode-field diameter of the desired mode in the waveguide is similar to the spot size associated with the fiber termination, and if the numerical apertures (NAs) of the fiber termination and waveguide are well-matched. However, in many practical applications, silicon waveguides must be relatively thin, having a thickness of less than 0.35 μm (with a numerical aperture (NA) essentially equal to 1) to remain single-mode in the vertical direction and enable high-speed electronic applications. By way of comparison, single mode fibers that are commonly used for telecommunications applications have mode field diameters ranging from 2.5–10 μm, with NAs ranging from 0.1–4.0. Therefore, it is clear that this edge coupling technique is not readily applicable for use with relatively thin, sub-micron dimensioned silicon waveguides.
As direct coupling in the above-described manner does not provide a sufficiently small spot size, alternative techniques to transfer light into a silicon waveguide have been developed. In one prior art technique, light is incident on a periodic grating structure that may be fabricated through conventional lithographic techniques. See, for example, Fundamentals of Optoelectronics, Chicago, Richard D. Irwin, Inc., by C. Pollock, 1995, at pages 309–320.
In an alternative prior art technique, an input beam is incident upon an optical element of high-index material that is disposed in very close proximity to the waveguide of interest. One exemplary arrangement of this technique is disclosed in an article entitled “Theory of Prism-Film Coupler and Thin-Film Light Guides”, by P. K. Tien et al., appearing in the Journal of the Optical Society of America, Vol. 60, 1970, at pages 1325–1337. In this context, “very close proximity” is intended to mean that the separation distance between the optical element and the waveguide permits evanescent coupling of light from the optical element to the waveguide. In order for evanescent coupling to occur, the medium separating the optical element from the waveguide must have a refractive index that is lower than those associated with the optical element and waveguide materials. In addition, the refractive index of the launch optical element must equal or exceed that of the waveguide material. In order to couple light efficiently from the optical element to the waveguide for a specified wavelength and waveguide thickness, light must be incident on the waveguide at a specific angle of incidence. To readily achieve the required angle of incidence, the optical element is frequently fabricated in the form of a prism. By varying the angle of incidence of the external beam on the angled facet of the prism, the beam inside the prism can be refracted at the desired angle. For this reason, the evanescent technique is generally referred to in the art as “prism coupling”.
FIG. 1 illustrates one such exemplary prior art prism coupling arrangement, wherein in particular FIG. 1 illustrates an exemplary optical waveguide device 1 such as disclosed and claimed in U.S. Pat. No. 6,526,187, issued on Feb. 25, 2003 and assigned to the same assignee as the present application. Referring to FIG. 1, an optical signal O passes through an input prism 2 and is thereafter coupled into a silicon optical waveguide layer 3, layer 3 being the upper silicon layer of a Silicon-On-Insulator (SOI) structure including a silicon substrate 4 and a buried oxide layer 5, layer 5 disposed between upper silicon waveguide layer 3 and silicon substrate 4, with layer 5 exhibiting a lower refractive index than silicon layers 3 and 4. A “gate” electrode 6 is disposed to cover the guided region above waveguide 3 between input prism 2 and an output prism 7. An upper insulating layer 8 (also referred to as the “gate oxide” and exhibiting a relatively low refractive index) is disposed between gate electrode 6 and waveguide layer 3, and is used to maintain light guiding within waveguide layer 3. It has been found that as a voltage potential is applied to gate electrode 6, the distribution of free carriers (either holes or electrons) near the boundary between waveguide layer 3 and gate oxide 8 changes, actuating an optical action in waveguide layer 3 sufficient to support propagation of an optical signal (the distribution change illustrated by the shaded region in FIG. 1). The position and physical properties of prisms 2 and 7 are utilized to couple the light signal into and out of waveguide 3.
It has been found, however, that the prisms are sub-optimal in terms of the amount of light actually coupled into the waveguide when the evanescent coupling region has a substantially constant thickness. Additionally, the quality of the prism edge has been found to be directly related to the uniformity (or lack of uniformity) in the coupled signal.
A known method of addressing this prism-to-waveguide coupling problem is to utilize a layer (that is, an evanescent coupling layer) of graded thickness between the prism exit surface and the entry point of the waveguide. By grading the thickness of the evanescent coupling layer, the shape of the output beam can be modified to improve the coupling into the silicon waveguide layer. To date, however, there are no known manufacturing methods or processes of producing precision input/output wafer structures with tapered evanescent coupling regions. Any such process requires a precise set of geometrical constraints on the coupling region, waveguide and prism coupling structure.
Thus, a need remains in the art for a robust method for providing a tapered evanescent coupling region that may be used to improve the coupling efficiency between an optical coupling prism (or any other external device) and an optical waveguide formed in an upper silicon layer of an SOI structure.